Cathode-follower and emitter-follower circuits



Dec. 29, 1964 K. KANDIAH 3,

CATHODE-FOLLOWER AND EMITTER-FOLLOWER CIRCUITS Filed Aug. 22, 1962 H 3 Sheets-Sheet 1 I VIN Dec, 29, 1964 K. KAN-DIAH 3,1

CATHODE-FOLLOWER AND EMITTER-FOLLOWER CIRCUITS Filed Aug. 22, 1962 3 Sheets-Sheet 2 Dec. 29, 1964 K. KANDIAH 3,163,827

CATHODE-FOLLOWER AND EMITTER-FOLLOWER CIRCUITS Fild Aug. 22, 1962 '5 Sheets-Shea 3 United States Patent 3,163,827 CATHGDE-FGLLGWER AND EMfi'TER-FGLLOWER CHKCUITS Kathirkamathamby Kandiah, Abingdou, England, assignor to United Kingdom Atomic Energy Authority, London, England Filed Aug. 22, 1962, Ser. No. 213,653 Claims priority, application Great Britain, Aug. 22, 1961, 30,279/61; Dec. 13, 1961, 44,703/ 61 4 (Ziairns. (Cl. 33(3-3) This invention relates to circuits for matching high impedance signal sources to low impedance loads and is concerned with cathode-follower and emitter-follower circuits.

The voltage gain of a simple cathode follower can be made to approach unity, which is desirable from the point of view of reducing the efiective input capacity, provided that the nett cathode load is of high impedance. A low output impedance can be obtained by using a White cathode follower but, in its normal form the low output impedance of the white cathode follower does not extend down to zero frequency because of the AC. coupling.

Emitter followers are also used to match high imped ance signal sources to low impedance loads. The output resistance is approximately s c E where r is the emitter resistance under common-base conditions, ,8 is the common emitter current gain and r is the resistance of the signal source. The output resistance is lower when two or more emitter followers are used in cascade but it can never be less than the r of the last stage, which is usually in theregion of a few ohms, unless power transistors are used with a large emitter current.

The present invention seeks to achieve a gain closely approaching unity and a low output impedance from zero up to a very high frequency.

. According to the present invention a circuit for matching a high impedance signal source to a low impedance .first transistor such that any change in the collector current of the first transistor is fed back to oppose the change, and an output connection from the emitter of the second transistor.

The transistors referred to as the first and second transistors are, of course, additional to the emitter follower transistor if one is used. They may be p-n-p transistors, and the collector load of the first trmsistor may be a resistor or a further transistor connected in a constantcurrent circuit.

Alternatively the first and second transistors may be n-p-n transistors, and the cathode load maybe a further transistor connected in a constant-current circuit.

Constant-voltage means may be connected between the output and the valve anode to maintain constant the anode-to-cathode voltage.

To enable the nature of the present invention and in what manner it is to be performed to be more readily understood, attention is directed by way of example to the accompanying drawings, wherein FIGS. 1, 2 and 3 are circuit diagrams of cathode-follower circuitsembodylarger than R1 so that 13R RL ing the invention and FIGS. 4 to 8 are circuit diagrams of emitter-follower circuits embodying the invention.

Referring firstly to FIG. 1, the cathode current of a valve V1 flows to the emitter of a first p-n-p transistor 32 and thence through the collector load R1. Any change in the collector current of 12 is fed to the base of a second p-n-p transistor J3 and fed back from J3 emitter to 12 base in a sense to oppose the change. The effect is to maintain the collector current of J2, and hence the cathode current of V1, substantially constant. Hence the emitterbase voltageof J2 and the grid-cathode voltage of V1 remain constant and the output from J3 emitter closely follows the input to the grid of V1. The transistor 13 drives the output load RL. The function of R2 is to provide current for positive excursions of the output.

It can be shown, to a good approximation, that where R 2 RL Consider first the simple case where R2 and RL are both Clearly R2 can be made large when the load resistance R1. is large. In this case:

Vin 1 1 1 If V1 has g=l0 ma./v., ,u=50 at a cathode current of 10 ma., the value of R1 will be 3K ohms. assuming the input to be at about zero volts.

Then in 1 o approx. 1+ 158 out Comparing results (3) and (4) it is seen that the loss of gain when the load resistance is reduced from a large value to ohms is about 2%. In the case of a conventional cathode follower using the same valve the loss of gain would be about 50%. A White cathode follower can give a performance similar to thatflof the circuit of FIG. 1, but not for very low signal frequencies except when elaborate DC. coupling is used.

An improvement can be obtained if we malte R1 very.

large by using a constant-current load in place of R1, since the whole of any change in J2 collector current is then fed to the base of 13. Equation 1 then reduces to:

.Fatented Dec. 29, 1964 than R the gain is almost unity as seen from (7).

'major advantage of this circuit is that the negative sup- The output resistance R; is then given by 1 ,u i R 6 0 9 l (u Using readily available valves and transistors the outfrequency up to a frequency at which J2 or J3 lose current gain. Typical component values in the circuit of FIG. 1 are RL= 1K ohms, Rl=3.3K ohms and R2=3OK ohms. E1 may be 150 volts positive and E2 may be 30 volts negative.

It is possible to obtain a voltage gain almost equal to unity as well as a low output resistance by using the improved circuit of FIG.'2. Here a constant-current collector load for J2 is provided by the n-p-n transistor J4, whose emitter current is defined by the potentiometer R5, R6 and the emitter resistor R7. In addition, the output voltage variations are fed to the anode of V1 through the gas discharge diode V2, thereby maintaining constant the anode-cathode voltage of V1 and hence the anode current. Zener diodes of the appropriate voltage can be used in place of V2 with advantage, because of their lower impedance. With these improvements Equation 1 simplifies to Where R is the value of the load resistance in parallel with the resistance R4- which is in series with the anode of V1. The output'resistance R is given by R approx. R (8) In generalit is possible to achieve an output resistance of about 1 ohm, and if the load resistance is much greater A ply voltage need only be slightly greater than the maximum negative excursion of the signal, because the collector of J4 need only be a few volts more positive than this supply. The emitter current of J3 for positive output excursions is supplied via V2.: Typical component values in the circuit of FIG. 2 are R4=3.3K ohms, R7=220 ohms, R5=2.2K ohms, R6=l5K ohms. V1 is an RCA Nuvistor 6CW4 and V2 has a running voltage of about 100v.

FIG. 3 shows a modification of the circuit of FIG. 2 for use with large positive excursions of load current. The p-n-p transistors J2, J3 are replaced by n-p-n transistors J2, J3. V1 has a constant-current cathode load comprising an n-p-n transistor J5 and associated resistors R12, R13 and R14. Any change in the cathode current of V1 therefore appears as a change of equal magnitude in the opposite sense in the emitter current of J2, whose collector current is fed back to the base or J2 via J3 as before to oppose the change. A constant-current load for the collector of I2 is unnecessary, since the positive supply voltage E1 is large (say 150 v.) and enables R8 to be large.v Typical component values in the circuit of FIG. 3 are'R4=3.3K ohms, R8=33K ohms, R9=1.5K ohms, R12=220 .ohms, R13=2.2K ohms, Rl4=15K ohms. V1 and V2 are as in FIG. 2. E5 may be 15 volts positive and E4 15 volts negative.

In some instances the transistor J2 or J2 may require a larger collector voltage than that provided in the circuits shown. This can be achieved by inserting a Zener diode of suitable voltage in series with the emitter of the output transistor J3 or J3.

In small-signal applications the circuits of FIGS. 2 and 3 can be particularly useful because of the extremely low input capacitance that can be achieved. Since the anode and cathode of V1 follow the input signal, the grid-anode and grid-cathode capacitances are effectively reduced by a'factor of about R/R The same reduction in the capacitance between the, input and various shields can be obtained by connecting these shields to the output. In practice it may be possible to reduce the input capacitance to a value considerably smaller than 1 p F. even when the load resistance at the output is as small as ohms.

When a large capacitance is placed across the output there will tend to be an overswing due to charge storage when handling fast transients. The use of the RC combination R15, C2 (FIG. 2) or R15, C2 (FIG. 3) shown will prevent this'overswing, by reducing the loop gain at high frequencies, R15 and R15 being in parallel with the collector loads of J2 and J2 respectively. It has been found preferably, however, to replace these overswing-prevention circuits C2, R15 and C2, R15 by CR integrating circuits connected in series with the input grid, in an analoguous manner to that shown in FIGS. 57 which are hereinafter described. Suitable values for the cathode-follower circuits are C=0.1 pf., R=lOOK ohms.- When the capacit'ative load is extremely large it is desirable to use a resistor (not shown) in series with the output in order to limit the peak currents in the transistors.

A basic emitter-follower circuit of the invention is shown in FIG. 4 in which an n-p-n transistor J1 connected as an emitter-follower takes the place of the valve V1. The current through both the transistor J1 and the pn-p transistor J2 is approximately E2/R1 and that through the p-n-p transistor J3 is approximately E1/R2 (neglect ing the current through the load RL) under static conditions. Most of the load current flows through J3. When an input signal voltage is applied of J1 base, the current through J1 and J2 tends to change but this produces a much larger change in the emitter current of J3 with the result that the final change in the current through J1 and J2 isconsiderably reduced. If V is the change in input voltage and V is the change in output voltage it can be shown that to a good approximation where r =emitter output resistance of J1 with common-base connection.

r =emitter output resistance of J2 with common-base connection.

r =emitter output resistance .of J3 with common-base It can be shown that, with the above approximations, the input resistance is plfigR shunted by r l where 13 is the common emitter current gain and r l is the collectorbase resistance of J 1. The output resistance is where R is the output resistance of the signal source. Thus when the load resistance is 100 ohms the input resistance will be 250K ohms shunted by r l assuming [3 :5 :50. The ratio out will be 1.004 assuming r =r ohm. resistance is 10 ohms the ratio When the load out is shown in FIG. 5. The current through J1 and J2 is fixed by a constant-current load which can take any of the well known forms. In the present embodiment this load is provided by the n-p-n transistor J4 whose emitter current is defined by the potentiometer R5, R6 and the emitter resistor R7 The Zener diode D ensures that the voltage between base and emitter of J1 remains constant so that r l of the first transistor no longer becomes an appreciable component of the input resistance. The current through R4 in excess of that through J1 flows through J3. The integrating time-constant R3C1 ensures that the system remains stable, it being necessary also to use as J2 as a transistor with a cut-ofi frequency much higher than that of J1 and J3. With J1 and J3 having cut-otf frequencies of about 1 or 2 -mc./s. it is desirable to use as J2 a transistor with a cut-oil frequency in excess of 50 mc./s. Typical values of R3 and C1 will then be 10K ohms and 47 pf. respectively. The addition of R3 and C1 does not appreciably affect the input impedance, C1 being very much smaller than the input capacity of the transistor. With this circuit, input resistances much greater than 1M ohm and out can be obtained when the load resistance is 1K ohm, using only a few milliamperes in the transistors. The output resistance will be less than 1 ohm.

FIG. 6 shows a further embodiment (analogous to the arrangement shown in FIG. 3) in which J2 is an n-p-n transistor, Constant-current loads 11 and 12 are preferably provided for J1 and J2 respectively, but resistors may be adequate in some applications. This circuit has two advantageous characteristics. Firstly, when the input voltage at the base'of J1 is zero the output voltage is Zero since the emitter-base potentials of J1 and J2' are in opposite directions and so substantially cancel out; hence the load current is zero. Secondly, because these potentials are in opposite directions, variations therein due to temperature changes similarly cancel out, so reducing drift. By contrast the emitter-base potentials of J1 and J2 in FIGS. 4 and 5 are in the same direction, so that when the input voltage is zero the output voltage is the sum of the emitter-base potentials of the two transistors below zero, i.e., about 0.4 v., and some current flows through the load; variations due to temperature changes are similarly additive, giving rise to drifts.

Another form of the circuit which is particularly useful for driving a low value of load resistance with peak voltages approaching the values of the power supply lines is shown in FIG. 7. In this arrangement the collector of J1 is taken to the base of a p-n-p transistor J 4 whose emitter is connected to the positive supply line and whose collector is connected to the output. In FIGS. 4 and 5 the their current (gains.

7 i 6 current available for the load during large positive'voltage excursions of the output is limited by R2 and R4 respectively. In the circuit of FIG. 7 the collector of J4 can rise to within 0.5 v. of the positive supply line +E1 without limiting thev current flowing through J4 to the load. It J3 and J4 have equal values of current gain the base current in each will be half that supplied bythe constantcurrent source I3. In general the base currents of the output transistors J 3, J4 will be inversely proportional to If the load resistance is extremely small the cur-rent in J3 will be greater than that in J4 when the input is at earth potential, since the emitter of J3 is slightly negative, as mentioned above, and some current will flow through the load. This circuit can be used to drive loads such as loudspeakers directly.

FIG. 8 shows a circuit for use where supply lines of both polarities are not available. One end of the load RL (which might be a loudspeaker) is connected to the output of a circuit as in FIG. 7 comprising J1, J2, J3, and J4. The other end of RL is connected to the output of an identical referencecircuit comprising J11, J22, J33

and J44. The input voltage V is applied between the bases of J]. and Jill, Whose static potential is determined by a tapping on a'common potential divider R19, R11 connected to J11 base.

The main virtues of the circuits are high input impedance, low output impedance and a gain very close to unit. All the above characteristics are maintained from DC. up to a frequency whose limit is set purely by the cut-off frequencies of the transistors.

In all the described circuits the n-p-n transistor J1 can be replaced by a p-n-p type if corresponding modifications are made to the remainder of :the circuit. For example in FIG. 4, J2 and J3 would then become n-p-n type transisters. a

I claim:

1. A circuit for matching a high impedance signal source to a low impedance load comprising a thermionic valve which has an anode, a cathode and a control grid and which is connected as a cathode-follower, an input connection to said control grid, first and second supply lines, a connection from said anode to the first supply line, a first junction transistor having its emitter connected to said cathode such that any change in the cathode current of said valve appears as a change of equal magnitude in the emitter current of the first transistor, a resistive collector load for the first transistor connected between the collector electrode of the first transistor and the second supply line, a second junction transistor, connections between the collector of the first transistor and the base of the second transistor and between the emitter of the second transistor and the base of the first transistor such that any change in the'collec-t-or current of the first transistor is fed back to oppose the change, a connection between the collector of the second transistor and the second supply line, a resistive connection between the base of the first transistor and the first supply line, and an output connection from the emitter of the second transistor.

2. A circuit for matching a high impedance signal source to a low impedance load comprising a thermionic valvewhich has an anode, a cathode and a control grid and which i connected as a cathode-follower, an input connection to said control grid, first and second supply lines, a resistive connection from said anode to the first supply line, a first junction transistor having its emitter connected to said cathode such that any change in the which is connected in series with resistance between said cathode and the, second supply line, and the base of which is connected to -a point on a potentiometer connected between the second supply line and earth, a constantvoltage device connected between said anode and the base of the first transistor such that the anode current of said valve is maintained substantially constant, and an output connection from the emitter of the second transistor.

3. A circuit for matching a high impedance signal source to a low impedance load comprising a first junction transistor which is connected as an emitter-follower, an input connection to the base of the first transistor, first and second supply lines, a connection from the collector of the first transistor to the first supply line, a second transistor having its emitter connected to the emitter of the first transistor such that any change in the emitter current of the first transistor appears as a change of equal magnitude in the emitter current of the second transistor, a re sistive collector load for the second transistor connected between the collector electrode of the second transistor and the second supply line, a third junction transistor, connections between the collector of the second transistor and the base of the third transistor and between the emitter of the third transistor and the base of the second transistor such that any change in the collector current of the second transistor is fed back to oppose the change, a connection between the collector of the third transistor and the second supply line, a resistive connection between the base of the second transistor and the first supply line, and

an output connection from the emitter of the'second transistor. 7 i i 4. A circuit for matching a high impedance signal source to a low impedance load comprising a first junction transistor which is connected as an emitter-follower, an input connection to the base of the first transistor, first and second supply lines, a resistive connection from the collector of the first transistor to the first supply line, a second transistor having its emitter connected to the emitter of the first transistor such that'any change in the emit ter current of the first transistor appears as a change of equal magnitude in the emitter current of the second transistor, a third junction transistor, connections between the collector of the second transistor and the base of the third transistor and between the emitter of the third transistor and the base of the second transistor such that any change in the collector current of the second transistor is fed back to oppose the change, a constant-current network formed by a fourth junction transistor the emittercollector path of which is connected in series with resistance between the collector of the second transistor and the second supply line, and the base of which is connected to a point on a potentiometer connected between the second supply line and'earth, a constant-voltage device connected between said anode and the base of the second transistor such that the collector current of the first transistor is maintained substantially constant, and an output connection from the emitter of the third transistor.

No references cited. 

1. A CIRCUIT FOR MATCHING A HIGH IMPEDANCE SIGNAL SOURCE TO A LOW IMPEDANCE LOAD COMPRISING THERMIONIC VALVE WHICH HAS AN ANODE, A CATHODE AND A CONTROL GRID AND WHICH IS CONNECTED AS A CATHODE-FOLLOWER, AN INPUT CONNECTION TO SAID CONTROL GRID, FIRST AND SECOND SUPPLY LINES, A CONNECTION FROM SAID ANODE TO THE FIRST SUPPLY LINE, A FIRST JUNCTION TRANSISTOR HAVING ITS EMITTER CONNECTED TO SAID CATHODE SUCH THAT ANY CHANGE IN THE CATHODE CURRENT OF SAID VALVE APPEARS AS A CHANGE OF EQUAL MAGNITUDE IN THE EMITTER CURRENT OF THE FIRST TRANSISTOR, A RESISTIVE COLLECTOR LOAD FOR THE FIRST TRANSISTOR CONNECTED BETWEEN THE COLLECTOR ELECTRODE OF THE FIRST TRANSISTOR AND THE SECOND SUPPLY LINE, A SECOND JUNCTION TRANSISTOR, CONNECTIONS BETWEEN THE COLLECTOR OF THE FIRST TRANSISTOR AND THE BASE OF THE SECOND TRANSISTOR AND BETWEEN THE EMITTER OF THE SECOND TRANSISTOR AND THE BASE OF THE FIRST TRANSISTOR SUCH THAT ANY CHANGE IN THE COLLECTOR CURRENT OF THE FIRST TRANSISTOR IS FED BACK TO OPPOSE THE CHANGE, A CONNECTION BE TWEEN THE COLLECTOR OF THE SECOND TRANSISTOR AND THE SECOND SUPPLY LINE, A RESISTIVE CONNECTION BETWEEN THE BASE OF THE FIRST TRANSISTOR AND THE FIRST SUPPLY LINE, AND AN OUTPUT CONNECTION FROM THE EMITTER OF THE SECOND TRANSISTOR.
 4. A CIRCUIT FOR MATCHING A HIGH IMPEDANCE SIGNAL SOURCE TO A LOW IMPEDANCE LOAD COMPRISING A FIRST JUNCTION TRANSISTOR WHICH IS CONNECTED AS AN EMITTER-FOLLOWER, AN INPUT CONNECTION TO THE BASE OF THE FIRST TRANSISTOR, FIRST AND SECOND SUPPLY LINES, A RESITIVE CONNECTION FROM THE COLLECTOR OF THE FIRST TRANSISTOR TO THE FIRST SUPPLY LINE, A SECOND TRANSISTOR HAVING ITS EMITTER CONNECTED TO THE EMITTER OF THE FIRST TRANSISTOR SUCH THAT ANY CHANGE IN THE EMITTER CURRENT OF THE FIRST TRANSISTOR APPEARS AS A CHANGE OF EQUAL MAGNITUDE IN THE EMITTER CURRENT OF THE SECOND TRANSISTOR, A THIRD JUNCTION TRANSISTOR, CONNECTIONS BETWEEN THE COLLECTOR OF THE SECOND TRANSISTOR AND THE BASE OF THE THIRD TRANSISTOR AND BETWEEN THE EMITTER OF THE THIRD TRANSISTOR AND THE BASE OF THE SECOND TRANSISTOR SUCH THAT ANY CHANGE IN THE COLLECTOR CURRENT OF THE SECOND TRANSISTOR IS FED BACK TO OPPOSE THE CHANGE, A CONSTANT-CURRENT NETWORK FORMED BY A FOURTH JUNCTION TRANSISTOR THE EMITTERCOLLECTOR PATH OF WHICH IS CONNECTED IN SERIES WITH RESISTANCE BETWEEN THE COLLECTOR OF THE SECOND TRANSISTOR AND THE SECOND SUPPLY LINE, AND THE BASE OF WHICH IS CONNECTED TO A POINT ON A POTENTIOMETER CONNECTED BETWEEN THE SECOND SUPPLY LINE AND EARTH, A CONSTANT-VOLTAGE DEVICE CONNECTED BETWEEN SAID ANODE AND THE BASE OF THE SECOND TRANSISTOR SUCH THAT THE COLLECTOR CURRENT OF THE FIRST TRANSISTOR IS MAINTAINED SUBSTANTIALLY CONSTANT, AND AN OUTPUT CONNECTION FROM THE EMITTER OF THE THIRD TRANSISTOR. 